South Sudan's President Salva Kiir said Tuesday that Khartoum had "declared war" on his country, as violence between the world's newest nation and Sudan intensified.

Kiir's comment -- in a meeting with his Chinese counterpart -- came after Sudanese warplanes bombed South Sudan's oil-rich border regions overnight, as violence persisted despite international calls for restraint.

The South Sudanese leader is in Beijing on an official visit aimed at shoring up ties with China.

The Asian giant has been a key ally and the largest economic partner of diplomatically isolated Khartoum.

"It (the visit) comes at a very critical moment for the Republic of South Sudan because our neighbour in Khartoum has declared war on the Republic of South Sudan," Kiir told China's President Hu Jintao.

Sudan's President Omar al-Bashir has threatened to crush the "insect" government of South Sudan, which provided some five percent of China's oil until it shut down production in January.

Landlocked South Sudan had been using a Sudanese pipeline and port to export its crude, but imposed the shutdown in a dispute over fees for the trans-shipments.

Kiir told Hu that he hoped China and the international community could offer "support and help" to his country, Chinese state television reported.

The Chinese president called on both South Sudan and Sudan to remain calm and resolve the conflict through peaceful negotiations, repeating earlier statements by China's foreign ministry.

"The top priority is to actively cooperate with the mediation efforts of the international community to stop the armed conflict in the border region," Hu was quoted as saying by the government broadcaster.

Beijing has repeatedly called for an end to weeks of border hostilities that saw the South seize Sudan's most important oil field in the Heglig area on April 10 for 10 days.

After heavy fighting that broke out last month, both Sudan and South Sudan are reportedly reinforcing troops numbers and digging into trenches along their contested border.

South Sudan said Sunday it had completed a withdrawal of its forces from the disputed area, but has warned it will fight back if Sudan does not end its aerial attacks.

The south's split from Sudan in July 2011 and recent clashes have forced a Chinese juggling act to maintain support for Khartoum while not alienating the south, the world's newest nation and source of most of former Sudan's oil.

On Tuesday, Beijing expressed concern over the recent escalation of tensions and urged the two countries to resolve their differences through dialogue.

"Ensuring sustainable oil production serves the common interests of the two countries as well as the interests of Chinese oil companies and their partners," said foreign ministry spokesman Liu Weimin.

Khartoum accuses the South of supporting anti-government rebels in Sudan and has rejected a return to the negotiating table. Its air raids on its neighbour have drawn condemnation from the United Nations, United States and France.

Li Guangyi, a professor at the Institute of African Studies at central China's University of Xiangtan, said oil would be the "main topic" of Kiir's visit, noting China is the biggest buyer of Sudanese oil.

Kiir will plead his nation's case to Beijing, Li said, but added China was unlikely to take sides and will continue pushing for dialogue.

"Fighting is not beneficial to (Sudan and South Sudan), but neither is it beneficial for China," Li said.

The current fighting is the worst since South Sudan won independence after a 1983-2005 civil war in which about two million people died.

Clashes have continued in recent days and Jonathan Holslag, of the Brussels Institute of Contemporary China Studies, said the situation showed "the limitation of China's traditional fence-sitting".

"With most of the oil in South Sudan and the best political partnership in the north, Beijing is in a hugely difficult situation and has not much leverage over Juba," he said.

Kiir, who will leave China on Saturday, opened South Sudan's first embassy in Beijing before meeting Hu, the official Xinhua news agency said.

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